The marking and subsequent cutting of mats of octagonal aperture design present difficulties. While four of the octagonal mat aperture sides are parallel to the outer rectangular margin of the mat, the other four sides of the octagonal aperture are each uniformly inclined at 45.degree. with the adjacent outer margin of the mat. Particularly where a second inter liner or inner mat is used, having a somewhat smaller octagonal aperture whose sides are parallel to the octagonal aperture in the outer mat, marking and cutting difficulties arise. While linear measure can be used (with the outer margin of the outer mat as a reference) to locate the sides of the mat apertures (inner and outer) parallel to the outer mat margin, when the diagonal or 45.degree. lines are marked, unless a mathematical calculation is made, a triangulation error is introduced (as will be subsequently explained in detail) which causes the width of the border between the inner and outer mat apertures to be narrower at the 45.degree. lines. Loss of symmetry of appearance resulting from this difficulty can make the mat unacceptable. Either increased scrap loss or increased time and computational skill in marking had to be tolerated, heretofore, to avoid this problem.
The instrument of the present invention, used with a conventional framer's T-square, provides a means for transferring the 45.degree. marking lines to the corner of a mat without applying a numerical value to the given border measurements of the mat and, further, avoids the triangulation error introduced when conventional measuring techniques are used in marking the inner one of dual mats.